Parah, Chapter 8, Mishnah 7. Our mishnah takes up a rule about the levels of tumah and taharah, and specifically about the special standing that liquids were given by the Chachamim. The principle it lays down is broad: anything at all that has the power to disqualify terumah has the power to raise liquids to a higher rung of tumah than the rung it itself occupies.
Listen to how the mishnah phrases it. It begins with "Kol haposel es haterumah": every source of tumah powerful enough to disqualify terumah. Of such a source the mishnah says "metamei es hamashkin", it transmits tumah to liquids, and it does so "liheyos techilah", making them a first-degree source of impurity. Those liquids in turn accomplish "uletamei echad velifsol echad", imparting one level of actual tumah and one level of mere disqualification. And the mishnah closes with a single reservation, "chutz mitvul yom", apart from a tvul yom, whose case it will explain at the end.
Whatever disqualifies terumah
What sort of thing is included in the phrase "whatever renders terumah unfit"? An item that carries tumah of the second degree, a sheni, which disqualifies terumah upon contact. Hands too fall into this category, since hands are treated as carrying second-degree tumah and therefore render terumah unfit when they touch it.
Liquids jump to the first degree
Now comes the striking part of the rule. Although this item itself is only a sheni, when it comes into contact with liquids the liquids do not become a shelishi, as one would expect: they become a techilah, a rishon, tumah of the first degree. The reason is a protective measure instituted by the Chachamim. Liquids that were rendered impure by a sheretz carry first-degree tumah by Torah law, and so that people should not come to treat impure liquids lightly, all liquids that contract tumah were placed at that same first-degree level.
One tumah and one pesul
Once those liquids are a rishon, the mishnah tells us how far their impurity travels: they convey "one tumah and one invalidation." Food that touches these liquids becomes impure in the second degree, a sheni, and that is the "one tumah." Should that food then touch terumah, the terumah becomes a shelishi and is thereby rendered unfit, and that is the "one invalidation." There the chain stops, for a shelishi cannot pass tumah along to produce a fourth degree.
The exception: a tvul yom
The mishnah excludes one case from the entire rule: a tvul yom. This is a person who had been ritually impure, immersed in a mikveh, but whose sun has not yet set, so his purification is not yet complete. Such a person does render terumah unfit, yet he does not raise liquids to the first degree of tumah. Even when he touches liquids of terumah he does not render them impure at all; he merely invalidates them.
The mishnah dramatizes the point by putting an argument in the mouth of the liquid, introducing it with the words "harei zeh omer", this one says: "metamecha lo timeuni, ve'atah timeisani?" The very source that made you impure had no power to make me impure, so how could you possibly make me impure? Having already gone down to the mikveh, the tvul yom occupies too weak a position to push liquids up to first-degree tumah; all that remains within his reach is to disqualify them.
The lesson of this mishnah, then, is a rule and its single exception: contact that disqualifies terumah lifts liquids to the first degree of tumah, with all that follows from it, but a tvul yom stands outside that rule entirely.